This invention relates to a ceramic plate which is coated with metal at least on one side, particularly to such plate which consists of a material having a hexagonal crystal structure. A conventional ceramic material having a hexagonal crystal structure is alumina.
The plates of alumina which are covered with a metal plate at least on one side and particularly on both sides are used, e.g., as heat sinks for electronic power components. For use under certain conditions, e.g., in igniting devices for spark ignition engines for automobiles, the metal-coated ceramic plates are required to have a high resistance to cyclic temperature changes. Owing to the different coefficients of expansion of the ceramic material and of the metal, temperature cycles will result in tensile and compressive stresses in alternation in the ceramic plate in a direction which is parallel to its metal-coated surface. In case of an inadequate resistance to cyclic temperature changes, the temperature cycles may result in a fracture in the ceramic plate in a direction which is parallel to the surface of the plate (conchoidal fracture). The risk of such fracture will be particularly high if the ceramic material and the metal have highly different coefficients of expansion, as will particularly be the case if a metal having a high thermal conductivity, such as copper, is joined to the ceramic plate directly, without an interlayer consisting of a different metal or of a metal alloy. An interlayer having a high ductility, such as an interlayer consisting of a solder alloy, could compensate the resulting thermally induced stresses in part.
Journal of the American Ceramic Society, Vol. 58, No. 9-10, pages 409-412, September-October 1975, contains a disclosure of an Al.sub.2 O.sub.3 substrate, which has a relative degree of texture of 0 to 3 measured in the 006-pole figure. In contrast to the present specification, however, the relative degree of texture as given in said publication is defined as the ratio of the relative degrees of texture in the 006- and 113- pole figures. It has also been disclosed that the sintering temperature has an influence on the texture. Journal of the American Ceramic Society, Vol. 65, No. 5, pages 149-158, March 1982, contains a description of a metal-coated ceramic plate and its use in power electronics and states that the resistance of metal-coated ceramic plates to cyclic temperature changes is adversely affected by thermally induced stresses. It is stated there that the resistance to cyclic temperature changes is to be improved by the selection of a special coating process.